1. Field of the Invention
The present invention generally relates to an air/fuel control system for automotive vehicles and, more particularly, to the air/fuel control system designed for a supercharged automotive engine to be operated with a combustible mixture having an air/fuel ratio that is higher than the stoichiometric ratio during a particular operating condition of the engine.
2. Description of the Prior Art
In an automotive power plant, it is generally well known that, although the supply of a lean air-fuel mixture, i.e., a combustible mixture having an air/fuel ratio higher than the stoichiometric value, to the automotive engine may bring about a reduction in the engine power output, it brings about a reduction in fuel consumption because of the substantially complete combustion of the mixture taking place in the engine and a reduction in NOx emission because of the lowering of the combustion temperature.
Based on this general notion, Japanese Laidopen Patent Publication No. 57-210137 discloses a so-called "lean-burn engine" provided with an air/fuel control system operable to provide a combustible mixture with an air/fuel ratio of, for example, 17 to 24 which is higher than the stoichiometric value (i.e., 14.7) during a normal practical drive condition when a high engine output is not required, thereby accomplishing the reduction in fuel consumption and also the reduction in NOx emission.
It has, however, been found that, during the practical drive condition of the engine when a relatively high amount of torque is required, the operation of the engine with the lean combustible mixture results in a shortage in engine power. Therefore, it is a general practice that, when the throttle opening attains a value greater than a predetermined value TV.theta..sub.1 as shown in the graph of FIG. 11 during the practical drive condition, the air/fuel mixing ratio of the combustible mixture is controlled to, for example, 13.7, which is lower than the stoichiometric value.
In the engine disclosed in the previously mentioned publication, since for a given torque to be produced, the degree of throttle opening is comparatively large when the lean combustible mixture is supplied to the engine, and therefore, the available range (TV.theta..sub.1 to WOT shown in the graph of FIG. 11) over which the throttle opening can be adjusted during the drive condition (enriched drive condition) in which the engine is operated with the supply of the enriched combustible mixture is so narrow that, under the engine operating condition in which the throttle opening is greater than the value TV.theta..sub.1, an abrupt increase in torque tends to occur with an increase in the degree of throttle opening. Because of this, when the throttle opening attains the value TV.theta..sub.1, the torque produced by the engine undergoes an abrupt change as indicated by the point P in the graph of FIG. 11 and torque shocks occur to such an extent that the automobile driver is made uncomfortable.
In order to substantially eliminate the possible occurrence of the torque shocks, a method has been contemplated in which a shift from the lower air/fuel mixing ratio to the higher air/fuel mixing ratio is slowed. However, since, as shown in the graph of FIG. 12, the NOx emission tends to be at a maximum when the air/fuel mixing ratio is within the range of 15 to 16, the slowing of the shift from the lower air/fuel mixing ratio (L.sub.1) to the higher air/fuel mixing ratio (L.sub.2), when the throttle opening is within the range of, for example, TV.theta..sub.1 to TV.theta..sub.2, may result the supply of the combustible mixture having an air/fuel mixing ratio within the range of 15 to 16 continuing for a substantial length of time. This means that during such a substantial length of time, the maximum NOx emission takes place, as indicated by a portion R in the curve G in FIG. 11.